1. Art Field
This invention relates to a multilayer thin film structure comprising a perovskite oxide thin film or a metal thin film.
2. Background Art
Electronic devices have been elaborated which are fabricated by forming various function films such as superconducting, dielectric, ferroelectric or piezoelectric films on silicon substrates or semiconductor crystal substrates, followed by integration. For example, for the combination of semiconductors with superconductors, SQUIDs, Josephson devices, superconducting transistors, electromagnetic wave sensors and superconducting wired LSIs are contemplated. For the combination of semiconductors with dielectric materials, LSIs having a higher degree of integration and dielectric isolated LSIs relying on SOI technology are contemplated. For the combination of semiconductors with ferroelectric materials, non-volatile memories, infrared sensors, optical modulators, optical switches and OEICs (opto-electronic integrated circuits) are contemplated. For devices harnessing piezoelectric films, acoustic surface wave devices, filters, VCOs and resonators have been contemplated.
To ensure optimum device characteristics and reproducibility thereof for these electronic devices, it is desired that the function films be of good crystallinity. With omnidirectionally oriented polycrystalline materials, it is difficult to provide good device characteristics on account of the disturbances of physical quantities.
Among typical function films used for ferroelectric films, there is a perovskite oxide thin film. When the perovskite oxide thin film is formed on an Si substrate, it is known to interleave a buffer layer, e.g., a stabilized zirconia thin film or a rare earth oxide thin film between the perovskite oxide thin film and the substrate, as disclosed in JP-A's 9-110592 and 10-223476 published under the name of the applicant. For instance, the above JP-A 10-223476 teaches that to form a perovskite oxide thin film having tetragonal (001) orientation, the stress caused by a misfit between the buffer layer and the perovskite oxide thin film is used. In the present disclosure, that a film is of (001) orientation, for instance, means that the (001) face is present substantially parallel to the surface of the film.
A perovskite oxide thin film having ferroelectricity has generally an axis of polarization in the [001] direction in the case of a tetragonal crystal system, and in the [111] direction in the case of a rhombohedral crystal system. In order that the perovskite oxide thin film, for instance, can function well as a ferroelectric thin film, it should preferably be oriented in the (001) or (111) direction. However, since the perovskite oxide thin film is likely to have tetragonal (110) orientation or tetragonal (101) orientation on the above buffer layer, it is difficult to obtain a film in which the tetragonal (001) face or rhombohedral (111) face vertical to the axis of polarization is oriented. For instance, BaTiO.sub.3 is likely to have (110) orientation because it tends to form BaZrO.sub.3 (110) on zirconia (001). PbTiO.sub.3, too, is likely to have (110) or (101) orientation on zirconia (001). In addition, PbZrxTi.sub.1-x O.sub.3 (PZT) is oriented on Y.sub.2 O.sub.3 (111), and CeO.sub.2 (111) in the (101) direction, as reported in Jpn. J. Appl. Phys. 37(Pt. 1, No. 9B), 5145-5149 (1998).
To obtain a perovskite oxide thin film oriented in the axis-of-polarization direction such as one of tetragonal (001) orientation or rhombohedral (111) orientation, it is important that the thin film be oriented in the direction corresponding to such orientation during thin film growth. For instance, when a PbTiO.sub.3 (001) oriented film is formed, it is required to oriente this film in the (100) direction during film growth, because the PbTiO.sub.3 film appears as a tetragonal crystal at normal temperature, but it appears generally as a cubic crystal that is a high-temperature phase during film growth. If the PbTiO.sub.3 film can grow as a cubic (100) oriented film, then it transforms to a tetragonal crystal in the process of cooling after growth, yielding a (001) unidiretionally oriented film or a 90.degree. domain structure film in which (100) orientation and (001) orientation coexist. Whether the PbTiO.sub.3 film appears as the (001) unidirectionally oriented film or the 90.degree. domain structure film is determined depending on the difference in the coefficient of thermal expansion between the film and the substrate, and a lattice constant difference between the film and the buffer layer.
On the other hand, when the perovskite oxide thin film is used as a primer or subbing layer for a function film, for instance, when a Pt (100) oriented film is formed using a BaTiO.sub.3 film as the primer layer, it is important that the primer layer be of (100) orientation. When a ferroelectric function film such as a PbTiO.sub.3 film is formed on an electrode layer formed using a conductive perovskite oxide material such as SrRuO.sub.3, it is important that SrRuO.sub.3 be of (100) orientation or (001) orientation so as to obtain a function film of (001) orientation. In these cases, it is important that the perovskite oxide thin film formed as the primer or electrode layer be oriented in the (100) or (001) direction in the process of growth.
When the perovskite oxide thin film is formed on an Si substrate with a buffer layer interleaved between them, therefore, means is now desired, which enables the growing perovskite oxide thin film to be easily oriented in the (100), (001) or (111) direction depending on its crystal system.
Incidentally, an electrode film is usually provided between the Si substrate and the function film. To obtain a function film of good crystallinity, the electrode film must be formed as an epitaxial film close to a single crystal. To meet such demand, the inventors have proposed in the above JP-A 9-110592 to provide a buffer layer including a ZrO.sub.2 thin film, a stabilized zirconia thin film, and a rare earth element oxide thin film, each of (001) orientation, on an Si single crystal substrate, form a perovskite layer of (001) orientation such as one made up of BaTiO.sub.3 on the buffer layer, and form on the perovskite layer a metal thin film of Pt, etc., serving as an electrode film. The perovskite layer is provided for the following reason. If the Pt thin film is formed directly on the ZrO.sub.2 (001) thin film, then the Pt (100) unidirectionally oriented film cannot be formed because Pt is of (111) orientation or appears as a polycrystal. This is because due to a large lattice mismatching between the ZrO.sub.2 (001) face and the Pt (100) face, Pt grows while using the energetically stable (111) face as a growth face rather than grows epitaxially while using the (100) face as a growth face.
However, the perovskite layer is awkward to form, and it is particularly difficult to form a homogeneous perovskite layer having a composition as designed. For instance, when a BaTiO.sub.3 thin film is formed on a Zr-containing buffer layer, a substance susceptible to (110) orientation, e.g., BaZrO.sub.3 is likely to occur, as previously stated. In the above JP-A 9-110592, an evaporation process wherein metal vapor is fed to the surface of the substrate in an oxidizing gas atmosphere is used so that a large-area yet homogeneous thin film is formed. When the BaTiO.sub.3 thin film is formed by this process, however, it is required to place the amount of evaporation of Ba and Ti under precise control because Ba:Ti=1:1 upon deposition of oxides on the surface of the substrate.
Therefore, if a (100) oriented metal thin film of good crystallinity can be formed without recourse to a perovskite layer formed by a prior art process, then great breakthroughs can be achieved in terms of cost reductions, quality improvements, and yield improvements.
One object of the invention is to provide means that enables a perovskite oxide thin film having (100) orientation, (001) orientation or (111) orientation to be easily obtained. Another object of the invention is to provide a multilayer thin film comprising a unidirectionally oriented metal thin film of good crystallinity.
Such objects are achievable by the inventions as defined below.
(1) A multilayer thin film comprising a buffer layer and a perovskite oxide thin film grown thereon, wherein: PA1 (2) The multilayer thin film according to (1), wherein said perovskite oxide thin film is an oriented film having one or two of (100) orientation, (001) orientation, and (010) orientation. PA1 (3) The multilayer thin film according to (1), wherein said perovskite oxide thin film is a (111) unidirectionally oriented film. PA1 (4) The multilayer thin film according to (1), wherein said perovskite oxide thin film is primarily composed of lead titanate, lead zirconate, or a solid solution thereof. PA1 (5) The multilayer thin film according to (1), wherein said buffer layer contains a rare earth element oxide, zirconium oxide, or a zirconium oxide with a part of Zr substituted by a rare earth element or an alkaline earth element. PA1 (6) The multilayer thin film according to (5), wherein an atomic ratio, R/(Zr+R), in said buffer layer is 0.2 to 0.75 provided that R represents a rare earth element, and an alkaline earth element. PA1 (7) The multilayer thin film according to (5), which further comprises a primer layer on a side of said thin film that faces away from said perovskite oxide thin film with said buffer layer interleaved therebetween, said primer layer containing zirconium oxide or a zirconium oxide with a part of Zr substituted by a rare earth element or an alkaline earth element, provided that when said rare earth element and alkaline earth element are each represented by R, an atomic ratio, R/(Z+R), in said primer layer is smaller than said atomic ratio, R/(Z+R), in said buffer layer. PA1 (8) The multilayer thin film according to (1), which is present on a substrate having a surface made up of a Si (100) single crystal. PA1 (9) A multilayer thin film comprising a metal thin film that is a cubic (100) unidirectionally oriented epitaxial film, and a buffer layer wherein a {111} facet plane is present on an interface of said buffer that is in contact with said metal thin film. PA1 (10) The multilayer thin film according to (9), wherein said buffer layer contains a rare earth element oxide, zirconium oxide, or a zirconium oxide with a part of Zr substituted by a rare earth element or an alkaline earth element. PA1 (11) The multilayer thin film according to (10), wherein an atomic ratio, R/(Z+R), in said buffer layer is 0.2 to 0.75 provided that R represents a rare earth element, and an alkaline earth element. PA1 (12) The multilayer thin film according to (10), which further comprises a primer layer on a side of said thin film that faces away from said metal thin film with said buffer layer interleaved therebetween, said primer layer containing zirconium oxide or a zirconium oxide with a part of Zr substituted by a rare earth element or an alkaline earth element, provided that when said rare earth element and alkaline earth element are each represented by R, an atomic ratio, R/(Z+R), in said primer layer is smaller than said atomic ratio, R/(Z+R), in said buffer layer. PA1 (13) The multilayer thin film according to (9), wherein said metal thin film contains at least one of Pt, Ir, Pd, and Rh. PA1 (14) The multilayer thin film according to (9), which is present on a substrate having a surface made up of a Si (100) single crystal.
an interface between said buffer layer and said perovskite oxide thin film is made up of a {111} facet plane, and PA2 substantially parallel to said facet plane there is present a {110} face of a cubic, rhombohedral, tetragonal or orthorhombic crystal of said perovskite oxide thin film, a {101} face of said tetragonal or orthorhombic crystal or a {011} face of said orthorhombic crystal.